The monk deeply venerated his God-bearing patron St. Hilarion the Great (October 21), and he strove to imitate his life, so he came to be called Hilarion the New. At the Dalmatus monastery, he was ordained presbyter. After the death of the igumen the brethren wanted to elect St Hilarion to this position, but learning of this, he secretly fled to Constantinople.

Then the monks of Dalmatus monastery sent a petition to Patriarch Nicephorus, asking that St Hilarion be assigned as igumen. The Patriarch summoned the saint and persuaded him to give his assent. St Hilarion submitted out of holy obedience. For eight years he peacefully guided the monastery, but in the year 813 the iconoclast Leo the Armenian (813-820) occupied the imperial throne. The saint refused to dishonor the holy icons, and he boldly accused the emperor of heresy, for which he endured many torments. They locked him up in prison for awhile, and vexed him with hunger and thirst.

The impious Patriarch Theodotus, who replaced the exiled Patriarch Nicephorus, caused the monk much suffering in demanding that he abandon Orthodoxy. The monks of the Dalmatus monastery went to the emperor and asked him to release the saint, promising to submit to the imperial will. After they returned to the monastery, however, St Hilarion and the monks continued to venerate the holy icons. The enraged emperor again threw the monk into prison. He gave the saint over to torture with all the means at his disposal, hoping to change his mind.

The wrath of God soon overtook the wicked emperor. He was cut down by his own soldiers in church at the very spot where he had once thrown down a holy icon. The new emperor Michael II (820-829) freed St Hilarion from his imprisonment, and the saint settled into a monastic cell. Upon the death of St Theodore the Studite (November 11), who also suffered for the holy icons, St Hilarion beheld holy angels taking the soul of St Theodore to Heaven.

Under the iconoclast emperor Theophilus (829-842), St Hilarion was again put under guard and beaten terribly, then they confined him on the island of Aphousia.

After the death of Theophilus, the holy empress St Theodora (842-855) gave orders to recall the confessors from exile. St Hilarion returned to the Dalmatus monastery, again agreeing to be igumen. He departed peacefully in the year 845.